1. Field of the Invention
The present invention relates to a projection optical system, exposure apparatus, and exposure method and, more particularly, to a projection optical system suitable for exposure apparatus used in producing such microdevices as semiconductor devices by photolithography.
2. Related Background Art
Currently, miniaturization has been promoted in production of semiconductor devices and semiconductor chip mounting substrates, and there are demands for a projection optical system with higher resolving power in the exposure apparatus for printing a pattern of a mask on a wafer as a photosensitive substrate. In order to satisfy such demands for high resolution, it is necessary to decrease the wavelength of exposure light and to increase the NA (image-side numerical aperture of the projection optical system).
However, the shorter the wavelength of the exposure light, the more limited the kinds of optical materials available for practical use because of absorption of light. In the case of the exposure apparatus using light in the ultraviolet region, for example, where the wavelength is not more than 200 nm, particularly, an ArF excimer laser beam (wavelength 193 nm) as exposure light, it is common design to make light transmitting members (e.g., lenses) in the projection optical system of silica and fluorite (calcium fluoride: CaF2).
Incidentally, if a silica optical component is located at a position with a high fluence (fluence=energy amount per unit area and unit pulse) in the projection optical system using the ArF excimer laser beam, it will be likely to suffer a local refractive-index change or compaction due to volumetric shrinkage, under irradiation with the laser beam, and, in turn, the volumetric change of the silica optical component can degrade the imaging performance of the projection optical system.
In the case of the exposure apparatus, the light transmitting member located near the photosensitive substrate in the projection optical system tends to have a small clear aperture radius and, in turn, the fluence tends to be high there. Therefore, where the light transmitting member located near the photosensitive substrate is made of silica, compaction is likely to occur in this silica transmitting member. As a result, the endurance of the projection optical system will depend upon the silica transmitting member being placed near the photosensitive substrate and likely to suffer compaction. Then, there is a known technology of forming the light transmitting member placed near the photosensitive substrate, of fluorite being an optical material free of compaction.
On the other hand, fluorite is a crystal material and has intrinsic birefringence. Particularly, for example, in a case where light with the wavelength of not more than 200 nm passes through fluorite, the influence of intrinsic birefringence is significant; therefore, it is necessary to suppress degradation of the imaging performance due to the intrinsic birefringence, by a combination of a pair of fluorite transmitting members with different crystallographic orientations. However, the degradation of the imaging performance due to the intrinsic birefringence cannot be completely suppressed even by the combination of a pair of fluorite transmitting members with different crystallographic orientations. Furthermore, it is known that an internal refractive-index distribution of fluorite has high-frequency components, and the variation of refractive index including the high-frequency components tends to cause a flare and thus to degrade the imaging performance of the projection optical system.
For example, in the case of the exposure apparatus using the ArF excimer laser beam as exposure light, degasification (outgassing) inevitably occurs from a photoresist during exposure. In the projection optical systems with relatively large numerical apertures proposed heretofore, therefore, contamination of lenses due to the degasification cannot be avoided without any particular countermeasures.
The present invention has been accomplished in view of the above problem and an object of the invention is to provide a projection optical system capable of securing good imaging performance over a long period, while avoiding the variation of refractive index including the high-frequency components and the influence of intrinsic birefringence of fluorite, for example, using the ArF excimer laser beam. Another object of the invention is to provide an exposure apparatus and exposure method capable of performing good projection exposure with high resolution, for example, using the projection optical system capable of securing good imaging performance over a long period, for example, for the ArF excimer laser beam.